The present disclosure arose from an effort to devise an air cleaning system for use in conjunction with containment buildings and facilities for nuclear reactors. The purpose of the specific air cleaning system was to limit release of aerosol particles and absorbable gases, including radioactive materials, from containment facilities during postulated major accidents. A system was devised which requires no energy while in the passive state, and no active energy other than pressurization of the stream of gas being scrubbed. While in its passive state, the system is instantly available for usage.
The disclosed system merges desirable features of both a pool type scrubber and a sand or gravel filter into a hybrid type of scrubber.
A pool type scrubber consists of a gas inlet duct projecting downwardly into a pool of liquid. Gas flows from the inlet duct into the pool of liquid, breaks into bubbles, and then flows upwardly through the pool. Aerosols are removed from the gas in the bubbles by various forces, which generally are very dependent upon bubble diameter. Although devices can be added to the outlet of the gas duct to reduce the bubble size, these systems characteristically produce relatively large gas bubbles and are subject to plugging by aerosol deposition. Aerosol removal is correspondingly low. However, pool type scrubbers have the desirable feature of being capable of handling a large mass of collected material as it is removed from the stream of gas.
Sand or gravel filters are constructed using layers of graded granular material, with the largest granular sizes normally positioned at the bottom of the filter bed and successive layers of smaller granules arranged upwardly from the bottom layer. Gas containing aerosols pass from the bottom to the top of the bed. Aerosol is removed by inertia, diffusion, interception and gravity forces. Because of the layers of fine sized granules at the upper portions of such a bed, sand or gravel filters demonstrate a high aerosol removal efficiency. However, because such a bed contains a limited void volume, the filter can handle only a small amount of collected material per unit volume of filter and then must be replaced, flushed or otherwise cleaned.
The disclosed hybrid scrubber consists of a porous bed at least partially submerged within a pool of liquid. The porous bed might contain granular material such as sand or gravel. The liquid might be water. A pressurized stream of air or other gas laden with aerosol is directed to the bottom of the bed. It distributes itself across the bed and flows upwardly through the irregular channels formed in the bed interstices. Aerosol is removed from the gas by interception, diffusion, inertia forces, particle growth, and settling. The porous bed itself is continuously cleaned by liquid entrained with the gas. The clean gas exits from the top of the porous bed.